This invention relates to a method of modifying polymers with a halogen or halogens; specifically to a process for the production of halogenated polymers. More specifically, it is directed to a continuous process for the manufacture of halogenated polymers such as butyl (a copolymer of a major proportion of an isoolefin and a minor proportion of a multi-olefin) EPM (a copolymer of ethylene and propylene), EPDM (a terpolymer of ethylene, propylene and a nonconjugated diene), SBR (styrenebutadiene rubber), BR (polybutadiene rubber), polyisoprene rubber, various types of polyethylene, including linear low density polyethylene, ethylene vinyl acetate copolymer, etc.; in particular halogenation wherein a substitution reaction results in the generation of hydrogen halide as a by-product or where hydrogen halide may be formed.
Numerous references teach methods for halogenating various polymers, for example the halogenation of butyl rubber. An early reference, U.S. Pat. No. 2,944,578, teaches that chlorinated butyl rubber can be produced in a batch process by dissolving butyl rubber in a suitable, nonreactive solvent, e.g., hexane, and introducing chlorine or a chlorinating agent. By suitable control of the temperature, concentrations of chlorinating agent and rubber, and reaction time, chlorinated rubber containing the desired level of chlorine is produced.
An improved, continuous solution process for chlorination or bromination of butyl rubber was subsequently disclosed in U.S. Pat. No. 3,099,644. The halogenation of ethylene-propylene nonconjugated diene elastomers (EPDM) has also been disclosed; such processes are analogous to those for halogenating butyl rubber. For example, U.S. Pat. No. 4,051,083 describes the solution bromination and chlorination of EPDM using N-halosuccinimide; additionally, the "neat" halogenation of EPDM is also described. In the latter disclosure the halogenating agent is dispersed in the EPDM by blending on a cool rubber mill and halogenation is effected by heating the mixture in a hydraulic press.
Halogenation of EPDM in an aqueous batch process is disclosed in U.S. Pat. No. 3,896,095. The process employs the addition of an excess of Cl.sub.2 or Br.sub.2 to a polymer slurry to effect halogenation and avoid the expense of solvent recovery systems previously disclosed for solution halogenation processes.
Chlorobromination of polymers such as polybutadiene, butadiene-isoprene copolymers and natural or synthetic polyisoprene is disclosed in British 1,483,063 and 1,483,064. The reaction is described as taking place at a low temperature of 0.degree.-15.degree. C., preferably in an inert solvent, and the halogenated products are described as containing high levels, e.g., at least 55% by weight of halogen.
The possibility of producing a halogenated rubber such as halogenated butyl rubber continuously in an extruder-reactor has been recognized, see, e.g., U.S. Pat. No. 4,185,057. However, the generalized disclosure of that reference does no more than acknolwedge the desirability of such a process, but does not teach one how to accomplish such a process. The reference suggests that only enough chlorine be introduced into the extruder to react with the butyl rubber so that no chlorine remains after reaction. It then goes on to suggest that another gas, e.g., nitrogen, be introduced to effect the production of gas filled pores in the finished rubber, which is the primary object of the invention. No examples are disclosed in the patent and no conditions disclosed which would enable one to actually conduct such a butyl halogenation process.
Chlorination of butyl rubber using dichloramine-T and a calender has been reported by Bulgarian workers (Kh. Tenchev. et al, Chem Abstracts 50756u). The disclosed process was not intended to produce neat chlorinated butyl since calendering is carried out on a mixture of butyl rubber, accelerators, prevulcanization inhibitors as well as variable amounts of carbon black and dichloramine-T.
The halogenation, in a kneader or extruder, of polymers containing carboxylic acid groups using reagents that differ from those disclosed herein is described in U.S. Pat. No. 3,364,187. The polymers are converted to the acyl halide derivatives using specific halogenating agents. The patent suggests that the kneading step may be carried out in an extruder, a Banbury mixer, a roll mill or any other apparatus that yields the described kneading action.
A British Patent, 1,257,016, discloses a process for treating polymers with halogenating agents such as N-bromosuccinimide under mechanical shear and at high temperature (120.degree.-180.degree. C.) for the purpose of producing unsaturation. The patent mentions that halogenation may possibly occur in an intermediate step followed by dehydrohalogenation, but production and isolation of a useful halogenated product is not an objective, nor is it achieved. The reference also discloses the use of scavenging amounts of a metal oxide or carbonate such as magnesium oxide, zinc oxide or calcium carbonate in addition to the halogenating agent and alpha-olefin polymer. The patent discloses, as an alternate method, the preblending of the halogenating agent with a solution of the polymer followed by solvent removal. It is stated that very little, if any, reaction occurs during such an operation. The reference does not disclose that scavenging agent can be used to produce halogenated polymers nor that the absence of such scavengers can affect the configuration of the halogenated polymer.
An extensive disclosure of polymer modifications conducted in an extruder can be found in U.S. Pat. No. 3,862,265. This patent is directed to modification of polyolefins using heat, shear and controlled pressure to induce degradation in the polyolefin and to combine the polyolefin with a free-radical initiator and/or one or more monomers. The broad disclosure is of value for its teachings directed to the modification of polyolefins with various monomers especially to form novel grafted polymers.
U.S. Pat. No. 3,510,416 (Vaccari et al) teaches an improved method of halogenating PVC particles by using gaseous chlorine in combination with a swelling agent (chlorination carrier). Following reaction, the PVC particles are transferred to another piece of equipment (a dryer) in which the chlorination carrier is stripped and gaseous by-products are separated. This reference discloses a process based on particle fluidization which relies on diffusion to accomplish drying; in addition, such a process requires separate pieces of equipment and relatively long times for drying.
A recent U.S. Pat. No. 4,384,072, and pending applications U.S. Ser. No. 481,366 (filed Apr. 1, 1983) and Ser. No. 481,320 (filed Apr. 1, 1983) disclose methods for halogenating neat polymers, i.e., not requiring the use of solution techniques. These disclosures are incorporated herein by reference for the benefit of their teachings and their relevance to this process generally, but they do not disclose the advance herein.
In 1979 Van Tongerloo et al disclosed a brominated butyl rubber which was low in conjugated diene content (if any) and in which the halogen was incorporated as primary allylic halogen as well as secondary and tertiary allylic halogen. (Van Tongerloo, A. and Vukov, R., Proceedings, International Rubber Conference, Milan, Italy, 1979, p. 70 ff). The reference states that the polymer was produced by a proprietary method and discloses only that rearrangement to the more stable primary configuration can be accomplished in brominated butyl rubber "under a variety of conditions--for example, in the presence of acid, free radicals, bases or heat."
Stabilized halogenated butyl rubber described in U.S. Pat. No. 4,130,519 discloses stabilizing agents which are added to the halogenated rubber following halogenation, i.e., in order to protect the rubber during further processing or during storage.
Some polymers, e.g., butyl rubber, are particularly sensitive when exposed to shear and elevated temperatures in the presence of a halogenating agent. The halogenation reaction of butyl rubber in solution is described in "Encyclopedia of Chemical Technology", Kirk-Othmer, Third Edition (1979), Volume 8 at page 476 ff. It is noted that the halogenation reaction carried beyond one halogen atom per olefin unit is complicated by chain fragmentation. Indeed, such fragmentation or degradation is a persistent problem when halogenation of butyl rubber is attempted; that problem is aggravated under conditions of heat and shear. A method of preparing halogenated butyl rubber in solution is described in U.S. Pat. No. 3,099,644 which is incorporated herein by reference.